Nematic liquid crystal display with surface bistability and control by flexoelectric effect

ABSTRACT

A bistable liquid crystal optical device includes two transparent plates (12, 14) provided with control electrodes (18, 19), between which is placed a nematic liquid crystal material (20). The transparent plates (12, 14) are surface-treated so as to define two stable configurations of molecules of the liquid crystal material generating respectively two flexoelectrical polarizations having components which are normal to electrodes P z  1P z  2 having opposite directions. Devices (30) are provided for applying perpendicular electrical field pulses to the plates (12, 14), said pulses being directed selectively one way or the other.

BACKGROUND OF THE INVENTION

The present invention relates to the field liquid crystal opticaldevices.

The present invention was made at the Laboratoire de Physique desSolides (Solid-State Physics Laboratory) of the Universite de Paris Sud(University of Paris South), which laboratory is associated with theCENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (NATIONAL CENTER FORSCIENTIFIC RESEARCH) number 04 0002.

Much research worked,has been conducted for at least the last fifteenyears or so on liquid crystals.

Various results of the research work carried out at the Laboratoire dePhysique des Solides de l'Universite Paris Sud are described in theFrench Patent Application filed on Apr. 28, 1982 under the No. 82 07309and published under the No. 2 526 177, the French Patent Applicationfiled on Oct. 23, 1984 under the No. 84 16192 and published under theNo. 2 572 210, the French Patent Application filed on Jun. 18, 1985under the No. 85 09224 and published under the No. 2 587 506, the FrenchPatent Application filed on May 14, 1986 under the No. 86 06916 andpublished under the No. 2 598 827 or again the French Patent Applicationfiled on Dec. 17, 1987 under the No. 87 17660 and published under theNo. 2 624 985.

Moreover, the work relating to liquid crystals has given rise tonumerous publications.

The present invention more precisely relates to the liquid crystaloptical devices which are called bistable, that is to say devices inwhich the molecules of the liquid crystals can alternately occupy twostable states, under the effect of external control. Such bistableoptical devices are suitable in particular for producing multiplexeddisplays.

Various liquid crystal bistable optical devices have already beenproposed.

The document Applied Physic [sic] Letters 40 (12) 1007 (1982) J. Chenget al. describes, for example, a nematic liquid crystal device havingtwo states exhibiting bulk stability switched by an external commandelectric field. The process described in this document has not givenrise to a practical application. It has a very slow commutation time andgenerally exhibits numerous textural defects.

The document Applied Physic [sic] Letters 36, 899 (1980), N. A. Clark etal. describes another bistable optical device using liquid crystalscalled Smectic C* ferroelectric, and degenerate surface anchorings. Theprocess described in this document has the advantage of a very shortswitching time and has given rise to practical applications. It is nothowever completely satisfactory.

In particular, in practice, it is frequently noticed that instead of adisplay arrangement which is bistable between two symmetric states,display arrangements which are monostable on twisted textures whosecontrast is poor and which cannot be multiplexed are obtained. Thisphenomenon seems to be due to the fact that the electrode/liquid crystalinterface is polar.

The document Applied Physic [sic] Letters Dec. 11, 1989, R. Barberi, M.Boix and G. Durand describes another bistable optical device in whichthe bistability is induced by a controlled toughening treatment on atleast one of the transparent electrodes and the switching is operated byapplication of an external electric field parallel to the electrodes.According to this document, the toughening treatment may be obtained,for example, by oblique evaporation of SiO. This Applied Physic [sic]Letter [sic] document is to be linked with the aforementioned FrenchPatent Application No. 87 17660. The process described in the documentApplied Physics Letters Dec. 11, 1989 seems promising. The specialistshave, however, always hitherto considered that this process has themajor drawback of being sensitive only to an electric field parallel tothe transparent plates of the device and of being completely insensitiveto an electric field perpendicular to the plates.

Another type of nematic bistable display using the bistability ofsurface orientation states, in which the switching controlled byelectric pulses of defined polarity is based on the use of chiral ions,is described in the French Patent Application filed on Jan. 30, 1990under the No. 90 01066.

SUMMARY OF THE INVENTION

The aim of the present invention is now to provide a novel liquidcrystal bistable optical device having better performances than theprior art.

An important aim of the present invention is to provide a liquid crystalbistable optical device with rapid switching, in particular forproducing highresolution multiplexed optical matrices.

Another important aim of the present invention is to provide a liquidcrystal bistable optical device designed in order to be controlledeasily by an external electric field.

These aims are achieved according to the present invention by virtue ofa bistable-effect liquid crystal optical device of the type comprisingtwo transparent plates provided with control electrodes and betweenwhich there is placed a nematic liquid crystal material, characterizedin that:

the transparent plates have a surface treatment capable of defining twostable configurations of liquid crystal material molecules respectivelygenerating two flexoelectric polarizations having components normal tothe electrodes of opposite directions, and

there are provided electrical supply means capable of applying to thedevice pulses of electric field perpendicular to the plates, orientedselectively in one direction or the other.

The alternate application of electric-field pulses normal to the plates,oriented in one direction then in the other, makes it possible to switchthe structure of the liquid crystal between the two stableconfigurations.

The effect of the control electric field will be specified in the restof the description.

According to another advantageous characteristic of the presentinvention, the device comprises electrical supply means designed inorder to apply to the device in succession:

at least one command pulse capable of inducing a generally homeotropichomogeneous orientation of the liquid crystal, then

a control pulse, of amplitude less than the command pulse, and ofpolarity chosen according to the required final state.

As will be explained in what follows, the command pulse may be thesubject of several variants. It is moreover of any polarity.

The use of command pulses and control pulses allows, in particular,simple control by multiplexing of a bistable nematic display.

For this purpose, according to an advantageous characteristic of thepresent invention, the optical device, in which the control electrodesare arranged in N lines and M columns defining a matrix of NM pixels attheir intersections, is characterized in that the command pulses areapplied successively on the N line electrodes, while at the end of eachcommand pulse, control pulses of respectively chosen polarity areapplied simultaneously on the whole of the M column electrodes.

The pulses necessary for the multiplexing of the liquid crystal deviceare thus much simpler than those used in the past, in particular for themultiplexing of the C* ferroelectric smectics.

Examples of command signals hitherto proposed for the C* ferroelectricsmectics are described in the following documents: 1) J. M. Geary,Proceedings of SID'85, pp. 128-130 (1985), 2) S. T. Lagerwall, J. Wahland N. A. Clarck, Proceedings of International Display ResearchConference, Ferroelectric Liquid Crystals for Displays, pp. 213-220(1985), 3) S. Shimoda, K. Ito, T. Harada, M. Taguchi, K. Iwara, M. Kai,Proceedings of Japan Display '86, pp. 460-462 (1986).

According to the present invention, the command pulse may be a singlesquare pulse, comprise two successive square pulses of polarity whichare opposite or alternatively comprise a high-frequency pulse train.

According to another advantageous characteristic of the presentinvention, the amplitude of the command pulses is between 1 and 100volts, typically between 10 and 20 volts, whereas the duration of thecommand pulses is greater than 1 μs, typically between 20 and 50 μs.

According to another advantageous characteristic of the presentinvention, the amplitude of the control pulses is between 0.1 and 10volts, typically between 0.1 and 5 volts, whereas the duration of thecontrol pulses is greater than 10 μs, typically between 25 and 50 μs.

The start of the control pulses may coincide with the end of the commandpulses.

In a variant, the start of the control pulses may precede the end of thecommand pulses.

It is advantageous for the control pulses to persist after the end ofthe command pulses for at least 10 to 50 μs.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics, aims and advantages of the present invention willemerge on reading the detailed description which is to follow and onconsidering the attached drawings which are given by way of non-limitingexamples and in which:

FIG. 1 represents a general schematic view of an optical deviceaccording to the present invention,

FIG. 2 diagrammatically illustrates the two stable configurations of themolecules,

FIG. 3 represents the orientation states of multistable surfaces createdby an oblique evaporation of directions e, for example of SiO, in FIG.3, P designates a planar stable orientation, O O' two oblique metastableorientations, [lacuna] a zenithal angle and [lacuna] an azimuthal angle,

FIG. 4 represents a perspective view of two electrodes having coplanarevaporation directions,

FIG. 5A represents a zenithal view of the orientations of an electrodeseen through the liquid crystal,

FIG. 5B represents the relative orientation of two electrodes turnedthrough a relative azimuthal angle of the order of 45°,

FIGS. 6A and 6B represent two different stable configurations of liquidcrystal material having flexoelectric polarizations of components normalto the electrodes of opposite direction,

FIG. 7 represents a typical recording of light transmitted through acell according to the present invention following the application of acommand pulse operating a switching of stable configurations,

FIG. 8 represents the command pulse threshold voltage as a function ofthe pulse duration for a display of 1 μm thickness,

FIG. 9 represents the command threshold field as a function of the pulseduration for cells of variable thickness,

FIG. 10 diagrammatically represents a first example of command signalsand of control signals according to the present invention,

FIG. 11 diagrammatically represents a second example of command signalsand of control signals according to the present invention,

FIG. 12 represents in tabular form the various states obtained as afunction of the command and control signals applied, and

FIG. 13 diagrammatically represents a matrix display controlled bymultiplexing according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS General Structure of the Device

The known basic structure of the optical device used comprises, asdiagrammatically represented in the attached FIG. 1, a cell 10 formed bytwo parallel transparent plates 12, 14, for example made of glass,separated by a wedge of constant thickness d (not represented in FIG. 1)and between which there is placed a nematic liquid crystal material 20.

The plates 12, 14 are provided on their facing internal surfaces, whichare adjacent to the liquid crystal, with electrically conducting andoptically transparent electrodes. Such an electrode is representeddiagrammatically in the form of a band 18, for the plate 14, in FIG. 1.The similar electrode provided on the plate 12 is labeled 19.

Electrical supply means 30 are connected between the electrodes providedon the two plates 12, 14 in order to apply a controlled electric fieldonto the liquid crystal material 20. These electrical supply means 30are advantageously designed in order to deliver electric pulses of aduration between 1 and 1000 μs and of amplitude between 1 and 100 volt,alternately with opposite polarities.

Essential Characteristics of the Invention

More precisely, the optical device according to the present inventionhas two essential characteristics:

the two transparent plates 12, 14 have a surface treatment capable ofdefining two stable configurations of liquid crystal material moleculesrespectively generating two flexoelectric polarizations havingcomponents normal to the electrodes of opposite directions, and

electrical supply means 30 are coupled to the electrodes 18, 19 so as toapply to the device pulses of electric field perpendicular to theplates, oriented alternately in one direction and then in the other.

The two stable configurations of the molecules of the liquid crystalmaterial correspond to two orientations of the molecules in planesorthogonal to the plates 12, 14 and have between them an azimuthalinclination of the order of 45°, so that when observed between analyzersand polarizers which are crossed, the cell thus formed appear [sic]alternately in the bright or dark state according to the configurationoccupied by the molecules of the liquid crystal material.

Various appropriate types of surface treatment will be described in whatfollows.

According to another advantageous characteristic of the presentinvention, the nematic liquid crystal used has a positive dielectricanisotropy.

Two stable configurations of the molecules of the liquid crystalmaterial generating opposite flexoelectric polarizations have beenrepresented diagrammatically in the attached FIG. 2.

In the attached FIG. 2 are again found the two transparent plates 12, 14between which there is placed the nematic liquid crystal material 20.

When the molecules of the liquid crystal material 20 are placed in afirst stable configuration, they are oriented along planes parallel tothe plane labeled 100 in FIG. 2. The plane 100 is orthogonal to theplates 12, 14. In this first configuration, the molecules of the liquidcrystal progressively pass from an orientation which is planar on theupper plate 12, that is to say parallel to this plate 12, to anorientation which is oblique, with zenithal angle θ₁, on the lower plate14.

When the molecules of the liquid crystal material 20 are placed in thesecond stable configuration, they are oriented along planes parallel tothe plane labeled 20 in FIG. 2. The plane 200 is orthogonal to theplates 12, 14. It has an azimuthal inclination φ of the order of 45°with respect to the aforementioned plane 100.

In the second configuration, the molecules of the liquid crystal 20progressively pass from an orientation which is planar on the lowerplate 14, that is to say parallel to this plate 14, to an orientationwhich is oblique, with zenithal angle θ₂, on the upper plate 12.

These two stable configurations generate flexoelectric polarizations P1,P2 whose components Pz1, Pz2 normal to the electrodes are opposite,directed respectively toward the lower plate 14 in the firstconfiguration and toward the upper plate 12 in the second configuration.

Operation

When a voltage is applied between the electrodes 18, 19 provided on theplates 12, 14, the molecules of the liquid crystal orientate themselvesperpendicularly to the said plates because of their positive dielectricanisotropy.

If the electrical excitation of the cell corresponds to a negative pulseon the upper electrode, at the end of the pulse, there remain near theupper electrode 19 positive ions and near the lower electrode 18negative ions which apply a depolarizing and transient electric fielddirected from the upper electrode 19 toward the lower electrode 18. Thefield promotes the creation of the first configuration which has aflexoelectric polarization component Pz1 also directed from the upperelectrode 19 toward the lower electrode 18.

After the relaxation time of the charges, the depolarizing fielddisappears and the first configuration is stable.

When the cell thus formed is observed between analyzers and polarizerswhich are crossed, the direction of the analyzer or of the polarizerbeing parallel to the plane 100, the cell appears black in the firstconfiguration.

If a positive pulse is then applied to the cell, the molecules alignthemselves perpendicularly to the plates 12, 14 during the pulse. At theend of this pulse, there remain near the upper electrode 19 negativeions and near the lower electrode 18 positive ions which apply adepolarizing and transient electric field directed from the lowerelectrode 18 toward the upper electrode 19.

This field promotes the creation of the second configuration which has aflexoelectric polarization component Pz2 directed toward the upperelectrode 19.

After the relaxation time of the charges, the depolarizing fielddisappears and the second configuration is stable.

The cell then appears bright.

In order to pass back into the first configuration, it is sufficient toapply a negative pulse to the cell.

Surface Treatment

As previously indicated, various types of surface treatment may be usedin the scope of the present invention in order to control the variousconfigurations required.

This surface treatment may be formed, for example, from the depositionof a polymer on the internal facing surfaces of the plates 12, 14followed by two abrasions of the polymer which are inclined to eachother, and according to methods known to the person skilled in the artin order to impose the desired obliqueness of the molecules on theplates.

According to another variant, the surface treatment may be formed bycontrolling the roughness of the surface of the plates 12, 14 (controlof the thickness of the roughness and of its mean incidence or meanwavelength) as taught in French Patent Application No. 87 17660published under No. 2 624 985.

Particular Embodiment

The structure and operation of a particular embodiment of the deviceaccording to the present invention will now be described.

I--DESCRIPTION OF THE DISPLAY a) Material

A nematic liquid crystal is used, for example 5CB, of dielectricanisotropy ε_(a) =ε.sub.∥ -ε.sub.⊥ >0 (ε_(a) is typically of the orderof 10). This liquid crystal may be doped with non-chiral ions ofsuitable concentration and with a cholesteric, as will be specified inwhat follows. This nematic is placed in a cell whose two ITO electrodesare treated in order to give bistable surface states.

b) Textures Used

Surface multistable orientation states are created by an evaporation ofSiO at an angle of 74°, of low mean thickness σ(σ, ≃30 Å). Preliminaryexperiments have shown that in these conditions, the stable molecularorientation state is a planar state P (molecules parallel to theelectrode, and perpendicular to the direction of evaporation asrepresented in FIG. 3), but there also exist two oblique metastablestates (O, O') of zenithal angle θ≃75° and of azimuth φ=±45 with respectto the direction of evaporation (see FIG. 3).

In the attached FIG. 3, the direction of evaporation is labeled e.

With respect to the publication "M. Monkade, M. Boix, G. Durand, Orderelectricity and oblique nematic orientation on rough solid surfaces,Europhys. Lett., 5, 697 (1988)", a situation Just below the degenerateplanar/oblique transition zone is found.

FIG. 4 represents in perspective the three surface states thus produced,one planar stable P and two oblique metastable O and O' on each of thetwo plates, in the case of evaporation direction [sic] which arecoplanar. The two electrodes 19 (top) and 18 (bottom) represented inFIG. 4 are identical.

FIG. 5A represents a zenithal view of the orientations of an electrodeseen through the liquid crystal.

As represented in FIG. 5B, the two electrodes are subjected to arelative rotation of 45° about an axis z which is normal to them.

The two textures of constant azimuth defined by the states P₁₉ O₁₈ andO₁₉ P₁₈ are used for the optical switching.

The aforementioned arrangement of the electrodes 18, 19 is defined bystarting from the symmetric situation in which the evaporationdirections are coplanar and the electrodes are facing, as represented inFIG. 4. One electrode (for example the electrode 19) is then rotated byφ₀ =45° so that the azimuths of the directions O₁₉ and P₁₈ coincide, aswell as those of P₁₉ O₁₈. The direction O₁₉ P₁₈ is then rotated by 45°with respect to O₁₈ P₁₉. In addition to these two textures of constantazimuth, there exist of course other textures of variable azimuth, whichare therefore twisted, for example O₁₉ P₁₈, P₁₉ P₁₈, etc. Thearrangement chosen is that for which these other textures have thehighest distortion energy, involving at the same time a change inazimuth φ and in zenithal angle θ. These high distortion energies makethese textures unstable. There nevertheless exists a texture of singletwist φ₀ between the states P₁₉ and P₁₈. In order to raise the energy ofthis state, it is possible to dope the nematic with a cholesteric sothat over the thickness d of the cell, the cholesteric-doped nematicrotates spontaneously in the opposite direction by an angle--φ₀ /2≃20°.This greatly increases the twist energy of the texture P₁₉ P₁₈, butwithout overly decreasing the energy of the undesired textures of typeO₁₉ O₁₈, etc. The twist--φ₀ /2=20° is obtained by fixing theconcentration of a cholesteric dopant. The Merck C15 cholestericmolecule has been used. For this body, at room temperature, the product:cholesteric pitch×concentration in 5CB is equal to 2 μm. In order toobtain 20° over the thickness d, a cholesteric pitch ≃18d is necessary,which fixes the concentration.

In conclusion, two bulk textures have thus been produced of constantazimuth rotated by 45° with respect to each other, while eliminating allthe others of too high curvature energy. These two states havespontaneous flexoelectric polarizations P=e[n(divn)+rotn×n] (e,flexoconstant, ≃10⁻⁴ cgs) whose components normal to the electrodes areof opposite directions. The flexoelectric polarizations are thereforeopposite with respect to the direction of the electric field applied asrepresented in FIGS. 6A and 6B. This polar property will be used for theelectrical switching between the two states.

c) Optical Assembly

The two textures O₁₉ P₁₈ and P₁₉ O₁₈ are placed between analyzers andpolarizers which are crossed. The texture O₁₈ P₁₉ is parallel to one ofthe directions of the analyzer or of the polarizer, and corresponds to astate which is extinct (black) in transmission. The other texture P₁₈O₁₉ is oriented at 45° . It restores the light if the resultingbirefringence corresponds to an optical delay of a multiple of λ/2(λ≃0.5 μm is the optical wavelength).

With a surface zenithal angle θ between 90° (for P) and 75° (for O), themean birefringence, corresponding to the mean angle θm≃(90+75)/2≃82°, isequal to: Δn=Δn0 sin² θm≃0.19 with Δn0≃0.2. The optimal thickness d_(o)defined by: d Δn=λ/2, gives:

    d.sub.o =(0.25/0.19)=1.3 μm.

In practice, in the experiment conducted by the inventors, d is fixed to1 μm by wedges of parylene C of the company Comelec. The two ITOelectrodes are held in a sample holder, in order to press correctly onthe wedges.

II--DESCRIPTION OF THE OPERATION a) Optical Observation

The pixel thus produced is observed under a Leitz polarizing microscope,whose polarizers are disposed as indicated hereinabove in Ic. The lighttransmitted is also observed by a photomultiplier and recorded on astorage oscilloscope.

The two electrodes are connected to a generator of a rectangular pulse,of duration τ(1 μs<τ<1 ms) and of constant amplitude V(-200<V<+200volt). The ground is connected to the electrode 18. In the absence ofelectrical excitation, one state O₁₉ P₁₈ or P₁₉ O₁₈ (or both states) isobserved at random. A positive pulse, of duration τ=100 μs, is applied.Below a threshold of V=+14 volt, a single bright state O₁₉ P₁₈ is madeto appear after the pulse. Another positive pulse does not change thestate. A negative pulse, V=-14 volt, is then applied which flips thestate O₁₉ P₁₈ toward the black state P₁₉ O₁₈. Another negative pulsedoes not change this new state. In the absence of a pulse, the twobright or black quasi-uniform textures of the pixel are stable forseveral hours.

b) Dynamics of the Switching

FIG. 7 shows a typical recording of the light transmitted through thecell. At the origin to of the recording, the cell is in the state O₁₉P₁₈ which transmits light. A -18 volt pulse of duration τ is applied att1. The light starts to fall at the start of the pulse and thencontinues for t=1 ms, in order finally to give the black state O₁₈ P₁₉.t is the well-known characteristic time of orientation of the bulkcurvature, in the absence of an electric field. t is defined by: t⁻¹=K/d² η where K is the constant of curvature of the nematic (K≃10⁻⁶ cgs)and η the viscosity (η≃0.1 cgs). t varies as the square of the thicknessd.

The inventors have measured the flipping threshold V(τ). The curvaturein FIG. 8 is obtained. For infinite τ(τ≃1 ms), V saturates toward±7.5volt. For shorter τ, it is necessary to increase V. It is found forexample for τ=64 μs (the access time of a line, for a video image with625 lines) that V=15 volt. For a 1000 line film speed, we have τ=40 μsand V=16 volt. τ=1 μs may be obtained for V=100 volt. These thresholdsare the same for the two switchings, bright-black and black-bright,between the two states.

c) Modelling

The inventors have measured the threshold field V(96 )/d for variousthicknesses d between 1 and 4 μm.

The curve obtained is represented in FIG. 9.

For large τ(>100 μs), the threshold V/d is constant, indicating that anelectric field effect is being observed. With short times (τ<20 μs, anincrease in the switching field, itself proportional to the thickness,is observed.

The switching mechanism is the following: the field effect at large τcorresponds to the breaking of the surface orientation. The field Etends to align the nematic normal to the electrodes (homeotropicorientation) because of the positive electric [sic] anisotropy of thecrystal. By symmetry, this state is an extremum of the surface energy.Defining the surface energy barrier between the two states by theextrapolation length L (0.1 μm<L<1 μm), this critical field is obtainedwhen the dielectric couple applied to the surface Just compensates forthe return couple of the surface orientation.

This gives the relationship: ##EQU1## where ζ is the electricalcoherence length defined by: ##EQU2##

We finally obtain ζ=L. With V=7.5 volt, it is found that L=(10⁻³ /E)=510⁻⁶ cm≃500Å, which corresponds to very strong anchoring.

At the end of the pulse, the homeotropic texture will again give atrandom one of the two textures O₁₉ P₁₈ or O₁₈ P₁₉. The degeneracy israised by the flexoelectric coupling between the textures and thedepolarizing field of the ions blocked at the surface.

Let us, for example, take for the explanation, the case of a negativepulse which causes switching from O₁₉ P₁₈ toward O₁₈ P₁₉. Just after thepulse, if the charges which determined the applied voltage V are removedfrom the electrodes, there remain near the top electrode 19 positiveions and near the bottom electrode 18 negative ions, which apply adepolarizing and transient electric field E_(d) from 19 toward 18. Thisfield promotes the creation of the P₁₉ O₁₈ state which has aflexoelectric polarization P_(z) also directed from 19 toward 18 with,therefore, an energy -P_(z) E_(d) which is favorable. The P₁₈ O₁₉ statehas an energy +P_(z) E_(d) which is unfavorable.

After the relaxation time of the charges, the field E_(d) disappears andthe P₁₉ O₁₈ state is stable, although having the same energy as the P₁₈O₁₉ state, because the system alone cannot get over the strong surfaceanchoring barrier.

With short times, it is necessary to increase V faster than thethickness, because another constraint appears: the ions must have thetime to be transported during the pulse τ from one electrode to theother.

Calling μ(μ≃10³ μm² /Vs) the mobility of the ions, their speed v isv=μE. A time of τ'=d/v=d² /μV≃130 μs is required in order to cover d=1μm with V=7.5 volt. Taking this transport condition as the onlylimitation, it is found that V/d=E≃d/μr', which is indeed proportionalto d as found experimentally.

If the two textures O₁₉ P₁₈ between which the switching is made hadexactly the same energy, an infinitesimal depolarizing field would besufficient in order to provide the switching. In fact, the two texturesare not exactly symmetric and have an energy difference ΔW. The minimumdepolarizing field E_(d) necessary is then that which gives aflexoelectric coupling energy ∫-E_(d) Pd³ r greater than |ΔW|. Inpractice, ΔW is not known. E_(d) is adjusted by doping the nematic withions. For 5CB, we have measured a resistance R of the sample with crosssection S=1 cm² and d=1 μm of R=1 to 10MΩ, depending on the initialpurity of the product. In order to increase the surface charges and thedepolarizing field, we doped 5CB with TBATPB (tetrabutylammoniumtetraphenylborate) in order to obtain a final relative molar ionconcentration of 10⁻³ to 10⁻⁵. We obtain the bistable effect within arange of sample resistance R=25kΩ to 1MΩ. In practice, the initialconductivity of the least resistant samples is sufficient to provide theeffect for the surface states which we used.

The bistable display thus formed on a pixel may be utilized inmultiplexed matrix screens. The surface time then defines the accesstime of the lines, and the bulk time that of the images. In the absenceof time constraint, the bistability of course allows infinitemultiplexing. In the presence of time constraint, it is possible totransmit a maximum number 1/τv of 1000 images/sec. The number of linesof these images is defined by the voltage used: 16 volt for 1000 linesat the video film speed for example (and 15 volt for 625 lines). Thissystem is therefore well adapted to the high-definition video displayarrangement.

VARIANT OF IMPLEMENTATION OF THE CONTROL SIGNALS 1. Reminder of thePreviously Described Embodiment

A method of control of the display was previously described whichconsists in applying onto the device a square electric pulse ofamplitude V (1 volt<V<100 volts) and of duration τ(1 μs<τ). This pulseorientates the bulk molecules normally to the electrodes (homeotropicorientation) and also breaks the surface orientation on the electrodes.

At the end of the pulse, the system can return to one of two stabletextures corresponding to different flexoelectric polarizations. The twostable textures are called A or B in what follows. In fact, the systemdoes not return at random. The final chosen texture depends on thepolarity of the pulse applied.

If the system is in the state A, a positive pulse for example, ofvoltage |V|>V_(s) (τ) or make it pass into the state B.

The application of another identical voltage of the same polarity leavesthe system in the state B.

The application of a negative pulse -V (|V|>V_(s) (τ)) flips the systemback into the state A. V_(s) (τ) is a threshold voltage dependent on τ.

The polarity of the effect is explained by the coupling between theflexoelectric polarization of the states A and B and the residual fieldin the cell when the pulse has Just been cut.

If the charges are removed from the electrodes, this residual field isthe depolarizing field created by the ions present in the cell, afterdoping, or not, of the nematic.

In practice, the voltage at the terminals of the cell is cancelledabruptly. There then remain charges out of equilibrium not only near theelectrodes (the ions), but also on the electrodes. The fielddistribution in the cell is more complex, but the sign of all the fieldsand of all the residual charges is strictly linked to the sign of theelectric pulse V applied.

The overall flexoelectric coupling of the residual field due to the ionsout of equilibrium near the electrodes and of the flexoelectricpolarizations of the nematic (and more generally of the depolarizingfield gradients and of the electric quadruple moments of the nematic)always promotes the return of the system toward that of the states A orB whose energy in the transient regime is the lowest, taking intoaccount the sign of the command pulse.

2. Subject of the Variant

The improvements made by the present variant relate to the transientphase of return to equilibrium when, after the surface orientation isbroken, the system has the choice of returning toward one of the twostates A or B.

It may appear that in practice, dependence on the concentration of theions in the cell for providing the switching toward one of the twostates A or B desired involves difficulties.

In order better to control this return, independently of the ionspresent in the cell, the inventors propose, as previously indicated,successive application to the device of: 1) at least one command pulsecapable of inducing a generally homeotropic homogeneous orientation ofthe liquid crystal, then 2) a control pulse, of lower amplitude than thecommand pulse, and of polarity chosen according to the required finalstate.

Art example of such successive command and control signals isrepresented in the attached FIG. 10.

This FIG. 10 shows a square command pulse Ca of voltage V (|V|>V_(s) (τ)as previously described applied at time 1.

The command pulse Ca ends at time 2. It lasts for time τ.

According to the present invention, the command pulse Ca is now followedby a second pulse Co: called a control pulse, of amplitude iv with 0.1volt ≦|v≦|<V_(s) (τ).

Typically, |v| is of the order of +5 volts.

The control pulse Co is maintained between the times 2 and 3, that is tosay during a time τ' with 10 μs<τ'<∞, typically τ' is between 25 μs and50 μs.

The control pulse Co allows the polarity of the field in the cell to becontrolled between the instants 2 and 3 when the system will switch fromthe homeotropic homogeneous orientation obtained by the command pulse Caof amplitude V at the instant 2, toward one of the states A or B ofdefined polarity.

By using an excitation with two successive, respectively command Ca andcontrol Co, pulses, as indicated in FIG. 10, the following effects areseen.

Let us assume the system to be in the state A, such that the applicationof a positive command pulse Ca of amplitude V makes it pass into B, inthe absence of control pulse v, as previously indicated.

The application of a control pulse Co of amplitude v>0 with v>v_(s) (τ')(v_(s) (50 μs)=3 volts) inhibits the flipping from A toward B. Thesystem remains in A.

Conversely, the application of a negative control pulse Co -v (whateverits amplitude), always promotes the flipping from A toward B.

In a symmetric manner, for a zero control pulse Co v=0, it is necessaryto apply a command voltage V<0 in order to switch from V toward A. Apositive control voltage Co v<0 always promotes the flipping from Btoward A, whatever its amplitude. In contrast, a negative controlvoltage Co -v<0 (|v|>v_(s) (τ') as previously) prevents the flippingfrom B toward A.

Finally, the state obtained after return to equilibrium depends only onthe polarity of the control pulse Co if the amplitude v of this controlpulse Co is chosen above the threshold |v_(s) |.sup.˜ 3 volts, and doesnot depend on the polarity of the command pulse Ca.

The means proposed by the present invention therefore allow thefunctions to be separated: the command pulse Ca of amplitude V breaksthe surface orientation, and the control pulse Co of amplitude v(|v|>v_(s) (τ')) controls by its sign the polarity of the final stage Aor B. The threshold v_(s) (τ') of the control pulse Co corresponds tothe compensation by this pulse v for the polarity controlled by theions. This threshold v_(s) falls when taking a liquid crystal which isless conductive. v>v_(s) (τ') means that the control pulse has an effectgreater than the depolarizing field of the ions. The switching tableobtained is given in FIG. 12 in which the useful states (|v|>v_(s) (τ'))are indicated in bold, and in which it has been assumed that |V|>V_(s)(τ).

It is seen on the table in FIG. 12 that if a control pulse Co is chosenof amplitude greater than the threshold (i.e. |v|>v_(s) (τ')), whichcorresponds to the states in bold characters, the final state dependsonly on the sign of the control pulse Co.

The "light-faced" states illustrated on the table in FIG. 12 correspondto the operation previously described with reference to FIGS. 1 to 9.

APPLICATION TO THE MULTIPLEXING OF A BISTABLE NEMATIC DISPLAY

The aforementioned means allow simple control of the display, bymultiplexing.

Let us assume as diagrammatically represented in FIG. 13, a matrixdisplay comprising N line electrodes referenced 18-1 to 18-N on a firstplate and M column electrodes referenced 19-1 to 19-M on the secondplate.

Each pixel defined by the intersection of a line electrode and a columnelectrode is identified by its coordinates i, j.

The method of multiplexing according to the present invention is thefollowing.

Each line 18-1 to 18-N, for example the line i, is opened in successionby exciting it with a command voltage Ca of amplitude V(|V|>V_(s) (τ))of arbitrary polarity. The command pulse Ca is applied onto the line i,according to the diagrammatic illustration in FIG. 13, the other linesreceive no signal (V=0). At the end of the excitation by the commandpulse V, the whole of the line i is erased, the molecules of the liquidcrystal take a homeotropic orientation.

Control pulses Co of amplitude ±|v|(v>v_(s) (τ')) are then sent inparallel onto all the columns M simultaneously, according to the desiredstate of the various pixels i, j (1≦j≦M) of the line i. The controlpulses Co of amplitude ±v are applied just at the end of the commandpulse Ca of amplitude V. The pixels i, j (1≦j≦M) of this line are thenplaced in the states A or B, depending on the sign of the small controlpulse v. The other lines which are not open (V=0) are insensitive to thecontrol pulse Co and keep their states A or B. For greater convenience,the application of the control pulse Co could be started before the endof the command pulse Ca. The requirement is that the control pulse Copersists 10 to 50 μs after the end of the command pulse Ca.

After the line i, the lines i+1, i+2, etc., will be opened insuccession, which lines will be erased and rewritten, in order to drawthe new image. Each line is therefore erased in succession by a commandpulse Ca of amplitude V during the time τ, and rewritten by a controlpulse Co of amplitude v during the time τ' which succeeds τ. The totaltime for erasing and rewriting a line is therefore, according to theaforementioned method: τ+τ', adjustable by varying the command pulse Caand the control pulse Co. The total time for erasing and rewriting acomplete image is then N(τ+τ').

However, the line i+1 may advantageously be opened with a command pulseCa during the time τ' of writing of the preceding line i with controlpulses Co. The total time for writing a complete image is then only N×τinstead of N(τ+τ').

In order to avoid the electrochemical effects, it is possible toalternate the sign of the command pulse V from one image to the other,or even replace the command pulse Ca of continuous amplitude V by acommand pulse Ca of high-frequency amplitude V. Such a high-frequencycommand pulse is represented diagrammatically in the form of twosuccessive pulses Ca1, Ca2 of opposite polarities, in FIG. 11. In thecase of a high-frequency excitation, the threshold v_(s) of the controlpulse Co is zero since there is no longer any polar memory of the ions.The amplitude v of the control pulse Co could be diminished. Theabsolute polarity of the states A and B depends on the sign of theflexoelectric constant of the nematic and of the angles of obliquenessof orientation on each electrode. It is of no importance in theoperation of the display, since it is sufficient to change the sign ofthe control pulse Co in order to promote one state or the other.

In conclusion, the bistable nematic display system, withflexoelectrically controlled surface bistability, may be multiplexedvery simply by virtue of the means proposed by the present variant.

For this purpose, each line electrode is sequentially excited by acontinuous command pulse Ca of amplitude V between 1 and 100 volts,typically 10 and 20 volts of duration greater than 1 μs, typicallybetween 20 and 50 μs, of arbitrary sign, or even a high-frequency pulse,which breaks the surface orientation and erases the line.

Just after the command pulse Ca, a control pulse Co of an amplitude vbetween 0.1 and 10 volts, typically of the order of 5 volts of durationgreater than 10 μs, typically between 25 and 50 μs, is applied inparallel onto all the columns. If the amplitude v of the control pulseCo is greater than the threshold required, (i.e. |v|>v_(s) (τ') of theorder of 3 volts), the final state of the pixels depends only on thepolarity of the control pulse Co.

The amplitude threshold v_(s) of the control pulse Co is zero for ahigh-frequency command excitation. In practice, a control pulse Cohaving an amplitude of the order of 0.1 volt may then be taken.

The sequential excitation of the lines is continued in order to scan thewhole image.

This method of multiplexing according to the present invention is muchsimpler than those previously proposed for the multiplexing of thesmectic C* ferroelectric smectics. For these latter, in fact, use isgenerally made of, in addition to an erasing pulse, a double writepulse, all these pulses (4 for example), being high-voltage pulses. Themethod according to the present invention uses only one "high" commandvoltage V, and a low control voltage ±v.

It will be noticed that the device according to the present inventionpresents no problem of electrochemical stability inasmuch as thecholesteric dopants possible used are electrochemically stable.

The invention is, of course, not limited to the particular embodimentswhich have just been described, but encompasses all variants inaccordance with its spirit.

We claim:
 1. Bistable-effect liquid crystal optical device comprisingtwo transparent plates provided with control electrodes and betweenwhich there is placed a nematic liquid crystal material whereinthetransparent plates have on inside surfaces a surface treatment fordefining two stable configurations of liquid crystal material moleculeson said inside surfaces of the transparent plates, said two stableconfigurations having different azimuthal orientations and differentzenithal orientations with respect to said transparent plates so as togenerate two flexoelectric polarizations having components normal to theelectrodes and of opposite directions, and there are provided electricalsupply means for applying to the device pulses of electric fieldperpendicular to the plates and oriented selectively in one direction orthe other.
 2. Device according to claim 1, characterized in that thenematic liquid crystal has a positive dielectric anisotropy.
 3. Deviceaccording to claim 1, characterized in that the nematic liquid crystalhas a positive dielectric anisotropy which is approximately equal to 10.4. Device according to claim 1, characterized in that the two stableconfigurations of molecules of the liquid crystal material correspond toliquid crystal textures with a constant azimuth for a givenconfiguration and an offset in azimuth from one configuration to theother.
 5. Device according to claim 4, characterized in that the twostable configurations of molecules of the liquid crystal materialcorrespond to textures offset by 45° to each other.
 6. Device accordingto claim 1, characterized in that one of the two stable configurationsof molecules of the liquid crystal material corresponds to anorientation which is planar on a first plate and oblique on a secondplate, and the other corresponds to an orientation which is oblique onthe first plate and planar on the second plate.
 7. Device according toclaim 1, characterized in that the liquid crystal cell comprising thetwo transparent plates is placed between a polarizer and an analyzerwhich are crossed.
 8. Device according to claim 1, characterized in thatliquid crystal material is doped with non-chiral ions.
 9. Deviceaccording to claim 1, characterized in that the relative molar ionconcentration is of the order of 10⁻³ to 10⁻⁵.
 10. Device according toclaim 1, characterized in that the two stable configurations areobtained by different abrasions which are inclined to each other andmade on the internal surface of the plates.
 11. Optical device accordingto claim 10, characterized in that two abrasions of a polymer depositedon the plates are at an angle of 45° to each other.
 12. Device accordingto claim 1, characterized in that the two stable configurations areobtained by controlling the thickness and the mean wavelength of theroughness on the internal surface of the plates.
 13. Device according toclaim 12, characterized in that the configurations are obtained bycontrol evaporation on the plates.
 14. Device according to claim 13,characterized in that the directions of evaporation on the two platesare relatively inclined of approximately 45° in azimuth.
 15. Deviceaccording to claim 12, characterized in that the configurations arecontrolled by evaporation of SiO under an angle of approximately 74°with a mean thickness of approximately 30Å.
 16. Device according toclaim 1, characterized in that the liquid crystal material is doped witha cholesteric.
 17. Device according to claim 1 wherein theconfigurations are obtained by control evaporation on the plates, thedirections of evaporation on the two plates being relatively inclined ofapproximately 45° in azimuth, and liquid crystal material is doped witha cholesteric the cholesteric defining a spontaneous rotation of thenematic liquid crystal material in an inverse direction to an offsetformed between the two directions of evaporation on the two respectiveplates and of amplitude of approximately half the azimuthal offsetbetween the two evaporation directions.
 18. Device according to claim 1,characterized in that the thickness of the cell is of 1.3 μ.
 19. Deviceaccording to claim 1, characterized in that the electrical supply meansare for applying command pulses of a duration between 1 and 1000 μs. 20.Device according to claim 1, characterized in that the electrical supplymeans are for applying to apply command pulses of an amplitude between 1and 100 volt.
 21. Device according to claim 1, characterized in that theelectrical supply means are for applying to the device in succession:atleast one command pulse for inducing a generally homeotropic homogeneousorientation of the liquid crystal, then a control pulse, of amplitudeless than the command pulse, and of a polarity chosen according to therequired final state.
 22. Device according to claim 21, in which thecontrol electrodes are arranged in N lines and M columns defining amatrix of NM pixels at their intersections, characterized in that thecommand pulses are applied successively onto the N line electrodes,while at the end of each command pulse, control pulses of respectivelychosen polarity are applied simultaneously onto the whole of the Mcolumn electrodes.
 23. Optical device according to claim 22,characterized in that the start of the command pulse on the lineelectrode i+1 coincides substantially with the end of the command pulseon the line electrode i.
 24. Optical device according to claim 21,characterized in that the command pulse is a single square pulse. 25.Optical device according to claim 21, characterized in that the commandpulse comprises successive square pulses of opposite polarities. 26.Optical device according to claim 21, characterized in that the commandpulse comprises a high-frequency pulse train.
 27. Optical deviceaccording to claim 21, characterized in that the amplitude of thecommand pulses is between 1 and 100 volts.
 28. Optical device accordingto claim 27, characterized in that the amplitude of the command pulsesis between 10 and 20 volts.
 29. Optical device according to claim 21,characterized in that the duration of the command pulses is greater than1 μs.
 30. Optical device according to claim 29, characterized in thatthe duration of the command pulses is between 20 and 50 μs.
 31. Opticaldevice according to claims 21, characterized in that the amplitude ofthe control pulses is between 0.1 and 10 volts.
 32. Optical deviceaccording to claim 31, characterized in that the amplitude of thecontrol pulses is between 0.1 and 5 volts.
 33. Optical device accordingto claim 21, characterized in that the duration of the control pulses isgreater than 10 μs.
 34. Optical device according to claim 33,characterized in that the duration of the control pulses is between 25and 50 μs.
 35. Optical device according to claim 21, characterized inthat the start of the control pulses coincides with the end of thecommand pulses.
 36. Optical device according to claim 21, characterizedin that the start of the control pulses precedes the end of the commandpulses).
 37. Optical device according to claim 21, characterized in thatthe control pulses persist after the end of the command pulses for atleast 10 μs to 50 μs.
 38. Bistable-effect liquid crystal optical devicecomprising two transparent plates provided with control electrodes andbetween which there is placed a nematic liquid crystal materialwherein,the transparent plates have on inside surfaces a surfacetreatment for defining two stable configurations of liquid crystalmaterial molecules, said two stable configurations having a constantazimuth for a given configuration, an offset in azimuth from oneconfiguration to the other, and different zenithal orientations withrespect to said transparent plates; one of the two stable configurationsof molecules of the liquid crystal material corresponding to anorientation which is planar on a first plate and oblique on a secondplate and the other corresponding to an orientation which is oblique onthe first plate and planar on the second plate, so as to generate twoflexoelectric polarizations having components normal to the electrodesand of opposite directions, and there are provided electrical supplymeans for applying to the device pulses of electric field perpendicularto the plates, oriented selectively in one direction or the other. 39.Bistable-effect liquid crystal optical device comprising two transparentplates provided with control electrodes and between which there isplaced a nematic liquid crystal material wherein,the transparent plateshave on inside surfaces a surface treatment for defining two stableconfigurations of liquid crystal material molecules, said two stableconfigurations having different azimuthal orientations and differentzenithal orientations with respect to said transparent plates so as togenerate two flexoelectric polarizations having components normal to theelectrodes and of opposite directions, and there are provided electricalsupply means for applying to the device in succession at least onecommand pulse for inducing a generally homeotropic orientation of theliquid crystal, then a control pulse of amplitude less than the commandpulse, and of polarity chosen according to the required final state. 40.Bistable-effect liquid crystal optical device comprising two transparentplates provided with control electrodes and between which there isplaced a nematic liquid crystal material wherein,the transparent plateshave on inside surfaces a surface treatment for defining two stableconfigurations of liquid crystal material molecules, said two stableconfigurations having different azimuthal orientations and differentzenithal orientations with respect to said transparent plates so as togenerate two flexoelectric polarizations having components normal to theelectrodes and of opposite directions, and there are provided electricalsupply means for applying to the device in succession at least onecommand pulse comprising a high frequency pulse train for inducing agenerally homeotropic orientation of the liquid crystal, then a controlpulse of amplitude less than the command pulse, and of polarity chosenaccording to the required final state.
 41. Bistable-effect liquidcrystal optical device comprising two transparent plates provided withcontrol electrodes and between which there is placed a nematic liquidcrystal material wherein,the transparent plates have on inside surfacesa surface treatment for defining two stable configurations of liquidcrystal material molecules, said two stable configurations having aconstant azimuth for a given configuration, an offset in azimuth fromone configuration to the other, and different zenithal orientations withrespect to said transparent plates; one of the two stable configurationsof molecules of the liquid crystal material corresponding to anorientation which is planar on a first plate and oblique on a secondplate, and the other corresponding to an orientation which is oblique onthe first plate and planar on the second plate, so as to generate twoflexoelectric polarizations having components normal to the electrodesand of opposite directions, and there are provided electrical supplymeans for applying to the device in succession at least one commandpulse for inducing a generally homeotropic orientation of the liquidcrystal, then a control pulse of amplitude less than the command pulse,and of polarity chosen according to the required final state. 42.Bistable-effect liquid crystal optical device comprising two transparentplates provided with control electrodes and between which there isplaced a nematic liquid crystal material wherein,the transparent plateshave on inside surfaces a surface treatment for defining two stableconfigurations of liquid crystal material molecules, said two stableconfigurations having a constant azimuth for a given configuration, anoffset in azimuth from one configuration to the other, and differentzenithal orientations with respect to said transparent plates; one ofthe two stable configurations of molecules of the liquid crystalmaterial corresponding to an orientation which is planar on a firstplate and oblique on a second plate, and the other corresponding to anorientation which is oblique on the first plate and planar on the secondplate, so as to generate two flexoelectric polarizations havingcomponents normal to the electrodes and of opposite directions, andthere are provided electrical supply means for applying to the device insuccession at least one command pulse comprising a high frequency pulsetrain for inducing a generally homeotropic orientation of the liquidcrystal, then a control pulse of amplitude less than the command pulse,and of polarity chosen according to the required final state.